Sugar making is one of the oldest industries in human history. One of the most important steps in the sugar making process is cane milling, which involves the grinding of sugar cane under pressure between counter-rotating rollers to extract the sucrose juice. A concise review of the cane milling technology is described in Cane Sugar Handbook, James C. P. Chen, 11th ed., John Wiley & Sons (1984). The materials contained therein are incorporated herein by reference. As taught by Cane Sugar Handbook, the most common milling units generally comprise three cylindrical mill rollers arranged in triangular form, although milling units with two to five or more rollers are also used. Usually three to seven sets of such mill units are used to form a milling tandem.
A mill roller typically comprises a cylindrical roll body tightly shrink-fitted upon a central shaft. In general, most mill roll bodies have V-shaped circumferential grooving on the periphery to increase the grinding area per unit length. The size of the grooving generally decreases from the first mill roller to the last mill roller and can range from four to six grooves to the inch to one inch per pitch or larger. Typically a three roller unit comprises a "top roller" (or "top roll") and two "bottom rollers" (or "bottom rolls") arranged in a triangular relationship. The two bottom rollers comprise a "feed roll" or "cane roll" at the upstream end for receiving the shredded cane, and a "discharge roll" or "bagasse roll" at the downstream end for exiting the crushed bagasse.
During the milling process the prepared cane is first fed into the opening between the top and the feed rolls. Then the bagasse, along with some expressed juice, is guided from the opening between the top and the feed rolls to that between the top and the discharge rolls over a curved plate positioned between the feed and discharge rolls below the top roller, frequently called a turn plate. The expressed juice is collected in a juice tray underneath the bottom rollers.
Due to continuous corrosion by the acidic sucrose juice and the heavy abrasion by the tremendous tonnage of cane that is processed under great pressure each day, all roll bodies inevitably experience noticeable wear as the cane harvesting season progresses. A reduction of the external dimensions by over an inch in one season is not uncommon.
The performance of a mill is often measured by three indications: (1) crushing or milling capacity, (2) sucrose extraction, and (3) bagasse moisture level; all except the bagasse moisture should be as high as possible. Unfortunately, one of the inherent operational difficulties experienced with the conventional rollers is the inadequate drainage of the expressed juice, a problem compounded by the common practice of adding water or thin juice to the bagasse to enhance the extraction, a process called "imbibition". Inadequate drainage can cause flooding at the entrance of the mill with the expressed juice sometimes flowing over the top of the top roller. It can lead to choking of the mill which seriously reduces the mill's crushing capacity. Inadequate drainage also aggravates the re-absorption problem, a phenomenon occurring when trapped juice near the top roll has to percolate its way through the cane blanket to the juice tray and when expressed juice at the pinch gets carried along by the expanding bagasse blanket extruding from the pinch opening. All such problems are detrimental to the performance of a mill.
Some of the drainage problems are ameliorated by using the so-called Messchaert juice grooves, which are essentially radial extensions of the bottoms of the V-grooves formed on the bottom rolls, especially on the feed rolls. The purpose of the Messchaert juice grooves is to provide outlets for the downward draining of the expressed juice. They are therefore of little or no benefit to the top rolls.
To further improve the drainage efficiency of a mill, a series of perforated rolls has been developed. U.S. Pat. No. 3,969,802 (hereinafter, "the '802 patent") discloses a perforated top roll which comprises a steel body with a plurality of peripheral grooves. A plurality of axially extending juice channels are provided within the roll body. Juice passages connecting the outer periphery and the juice channels are formed by first machining out a plurality of female threaded holes on the roll body surface. Then a plurality of male threaded plugs, or inserts, each containing a round radial perforation, are screwed into the female threaded holes. With continuous rotation of the roller and corrosion by the acidic sucrose juice, the threaded connection can become loose and eventually these inserts or plugs may fall out of the roll body, causing serious processing difficulties and equipment damage.
U.S. Pat. No. 4,391,026 ("the '026 patent") was intended to be an improvement over the '802 patent. It discloses a mill roll which similarly includes a roll body, a plurality of peripheral grooves, and a plurality of channels extending axially through the roll body at positions inwardly of the grooves. Perforations between the grooves and the channels are provided by forming within the roll body at the radial bottoms of the grooves a plurality of radial recesses and fitting within such recesses a plurality of inserts, each of such inserts containing a radially extending perforation. These inserts are then welded into the recesses. Such welds are often degraded by the acidity of the sucrose juice and the heavy abrasion and wearing of the roll surface, leading to the same insert fall-off problems and the related maintenance inconvenience.
U.S. Pat. No. 4,561,156 ("the '156 patent") discloses a roller comprising a plurality of roller shell segments, each roller shell segment having a plurality of peripheral grooves and ridges on the outer side and a longitudinal key on the inner side to fit a mounting sleeve. Juice collecting ports are provided within the roller shell segment to provide communication between the outer periphery and internal channels formed between the roller shell segments and the mounting sleeve. The mill roller of the '156 patent contains inserts that are quite different from those stated above; the entire roller shell segments are inserted onto the mounting sleeve by cap screws or other threading means. The entire insert segments can fall off from the roll body and cause more severe problems.
U.S. Pat. No. 4,765,550 ("the '550 patent") discloses a juice extracting mill roll provided with a plurality of juice channels connected with a plurality of juice inlet passages which extend to the periphery of the mill roller. The '550 patent distinguishes from the '802 and '026 patents in that the juice inlet passages have a longer dimension in an axial direction and a shorter dimension in a circumferential direction. The main object of the '550 patent is to reduce the risk of clogging of the juice inlet passages by bagasse and of the flow back of expressed juice from the juice channels to the periphery.
Other perforated mill rolls are disclosed, for example, in U.S. Pat. Nos. 4,546,698 and 4,989,305 and Australian Patent No. 556,846, all of which involving inserts that are fitted into recesses in the roll body from its outer periphery. These inserts are needed in order to provide radially inwardly diverging juice passages between the periphery of the roll body and the axial juice channels designed to facilitate flushing of trapped bagasse. However, none of these prior art patents addresses the issue of fall-off problems associated with such inserts. Because the inserts are fitted radially inward from the outer periphery of the roll body, the dimensions of the recesses are such that their cross-sectional areas cannot increase in the radially inward direction, and no structural means is available to keep the inserts in the recess.
Welding means provides a stronger securing force than threading means for holding the inserts in the mill roll. However, welds can be degraded by the corrosion of the acidic sucrose juice and externally applied welds are always at risk of being completely removed by the abrasion and wearing of the roll surface. Moreover, because cast iron objects are not as easily and readily weldable into other objects as steel, both the inserts and the roll body often have to be made of cast steel, even though it is well known in the art that cast steel has inferior resistance to corrosion and abrasion compared to cast iron.